Audio transmission network



1 July 3, 1945.

R11. HoLLlNGswoRi-H AUDI() TRANSMISSION NETWORK Filed may 14, 1942 2sheets-sheet 1 k 1 r Rw 1.Y 1 1 1 mw M 1 1 NN n R m m m kN ...m 1 lo1mm@ u Q 1 1 11 1 1 n H u 1 1E H u U f1 u.. m 1 1 1 WN U Fl 1 f 1 11 ,1m. H MEW l M MEW MFV 1 1\ 15H. 1 k ww 1 m. .www1 uf w1 1 1 11\vhwxmwummw\ 1 MN .Q 1 1 1 1) A V 1 1 1 1 k 1 1 1 2S. YNMNMQNM \N\ M 1EQYNMQ 1 Q 1 1 Patented July 3, 1945 nsrws;

AUmo TRANSMISSION Ns'rwoax mme noumnwnrth, Riverhead N. Y., mima toRadio Corporation of America, a corporation of Delaware Applicaties May14, 1942, sei-nu No. 442,894-

t 10 Claims.

v My present invention relates to audio transl .f mission networkarandmore particularly to a method of, and means for, resinserting generatedharmonics to improve narrow band recep- In the transmission ofintelligence -over wires or radio, it would be advantageous oftentimesto reduce the band width of transmission, or of reception, thereby toavoid interference and higher audio frequency noises, as for example,tube hiss.

The band of transmission, or of reception, can

be reduced only to a certain degree without impairing theinteliigibility'l of the modulation signal, should the latter be voiceor music.

It may. therefore, be stated that it is one of the main objects of mypresent invention to provide a method of modulation signal transmissionwherein the intelligibility of the modulation signais is reduced to apoint where the intelligence sought to be transmitted is impaired; apredetermined higher portion of the transmitted modulation signal beingsubjected to a distortion acteristlc of my invention are set forth with"process thereby to produce harmonics, and the Vharmonics being utilizedin the transmitted modulation signal energy to improve, the aforesaidintelligence. It has been experimentally demonstrated that i deiiniteimprovement of intelligibility can -be had by receiving along-distance.'short-wave telephone signal .on a' 4 kilocycle (kc.) bandin the intermediate frequency.' (I. F.) amplifier; the higher portion ofthe received band of audio frequencies being distorted to produceharmonics and the resulting harmonics being re-inserted in the audiofrequency'lenergy to give considerably improved intelligence and at thesame time avoid strong local interfering signals. Standard broadcaststation signals, such as are used in vthe 550-1'100 kc.range, are notfull range, and they too can be improved by the present method ofharmonic distortion and re-insertion thereby to extend the apparentaudio frequency range. The method is, also, applicable tophonograph-reproutilizes an audio. ampliner, a connection being made tothe ampliiier input to excite an auxiliary Y amplifier, and the outputof the, latter beingl passed through a high-pass `filter to excite agenerator of distortion harmonics with voltagepr'oduced by the higheraudio frequencies; there being utilized a path between the harmonicgenerator and the audio transmission' network wherebyV to restore to adesired degree portions of the harmonic content of the originalaudiofrequency signals lost either in transmission orreception.

The novel features which I believe to be charparticularity in theappended claims; the invention itself, however, as to both itsorganization and method of operation will best be understood byreference to the following description taken in connection with-thedrawings, in which I have indicated diagrammatically several circuitVorganizations whereby my invention may be' carried into eii'ect. i A

In the drawings, Fig. 1 shows an embodiment of the invention; Fig. 2illustratesamodification; Fig. 3 illustrates various characteristics oithe present system. v

Referring now to the accompanying1 drawings, there is shown in Fig. 1 anaudio transmission system which embodies an audio'transformerl a0 whoseprimary winding may be connected to any stood that the invention may beapplied to freduction where the space on the recording disc is limitedand does not permit ruil range recording. Similarly.. the method may beapplied to sound-on-illm recording, where the speed of the iilm will notpermit full range recording, as for is to provide anaudio transmissionsystem which 55 voice or music.

quency .bands lying above the'audio range. Furthermore, the modulationsignal source itself may be the detector output resistor ot aradiovbroa'dcast receiver operating in the 550 to 1600 kc. band. 0r, ii'desired, the receiver may be a frequency modulation receiver operatingin the assigned 4&-50` megacycleband. Furthermore, the modulation may bethedetector output ora iac-` `simile orv television receiver. v Again,the signal source .can be a,` microphone used in recording.

The source ot modulation la'nai energy can be a microphone used insouncl-on-illmA recording,

where the speed of the iilm will not permit full range rtording. For thepurpose of speciilcgillustration, let it be assumed that the signalsource provides a band of audiov frequencies, such as The secondarywinding of the transformer is connected to the input electrodes of anaudio frequency amplifier tube 2. The audio ampliner tube is ofconventional construction and connections, as those skilled in'the artwill readily recognize. The plate circuit of amplifier tube 2 includesthe primary winding of the audio transformer 3. The secondary winding ofthe transformer 3 is connected to any desired modulation utilizationnetwork. The nature of the utilization network will 'depend on thesystem in whichthe amplifier is used. It may be a reproducer of theloudspeaker type in the case of radio reception; or it may be arecording device; or it can be a television reproducer. The numeral I2designates the output leads to the utilization network.

It is tobe understood that-the network I-2-3 as shown has the variouselements thereof constructed and designed to provide a high fidelityaudio transmission line. n formers are chosen so that audio frequenciesof th'e entire audio frequency range can be transmitted faithfully,Those skilled in the art are fully acquainted with the manner in whichsuch high fidelity audio transmission is secured. To provide theharmonics to be re-inserted into the transmission line, a potentiometer4i has its re- I .sister connected across the primary winding oftransformer I. The amplifier tube 6 has its input electrodes coupled toth'e secondary winding of an audio transformer 5, and the primarywinding of the latter is connected across an adjustable portion of thepotentiometer resistor.

In other words, the adjustable slider of the potentiometer d isconnected to one end 'of the primary winding of transformer E. The platecircuit of ampliiier 8 includes the primary winding of transformerl. Theplate output transformer l is peaked by the retardation coil E wherebythe .transformer l can assume the characteristics of a high-pass filter.In other words the coil 8,Y which is a choke coil whose inductance canbe varied as indicated by the arrow, functions to adjust. the pass bandcharacteristic of transformer I so that a selected portion of the higheraudio frequencies can be transmitted to the input electrodes ofthe tube9.

Tube. 9 is shown as of the screen grid type. The signal grid bias is sochosen, and the magnitude of the vpositive voltage applied to the screengrid and plate is such, that tube 9 functions to distort the audio waveforms applied thereto. The tube can be operated class B or C, or in anyother manner which yields rich harmonics. The output voltage developedby tube 9 is rich in harmonics. Itis not believed necessary to describethe construction of the harmonic generator in further detail. Thoseskilled in the art are fully acquainted with the manner of constructinga tube circuit so as to distort audio waves supplied theretothereby toprovide abundant harmonics in the output circuit.

The harmonic energy is fed through the transformer Ill along with th'efundamental frequencies which were supplied through transformer 1. Thecombined harmonic energy and fundamental frequencies are fed into theoutput leads I2. A phase reversing switch II is provided so that theharmonic energy can be fed into the output circuit in proper phaserelation. The reversing switch neednot be used, if so desired. It willbe realized th'at the fundamental frequencies developed across theoutputof transformer I can be filtered out by using an additionalhigh-pass filter between transformer I0 and switch I I. Such 'Ihat is,the trans-- a filter would permit only the harmonic frequen# cies to befed to the output connections I2.

In order to prevent the harmonics from becom- Ping too strong duringhigh transmission levels, as

would ordinarily be the case in the latter condition of audio amplitude,there is employed a control circuit whose inputleads I3 are connected tothe opposite ends of th'e secondary winding of transf former 3. Theaudio energy. amplified by tube 2,

There will be developed across resistor I6 a direct;l

currentvoltage whose amplitude is dependent upon the percentagemodulation, or input signal energy level. In other words, during highamplitude levels of audio transmission, the voltage developed acrossresistor I6 `will have a greater magnitude.

It is only necessary to provide the direct current lvoltage connectionIl to a point on resistor I6 which becomes increasingly negative withrespect to the grounded cathode end of the resistor as the transmissionamplitude level increases. The end of lead Il connected to resistor I6may be made adjustable, and the other end of. the lad Ii is connected tothe lower end of the secondarywinding oftransformer l. It will now beseen that the signal grid of tube 3, the harmonic generator, is

variably biased in dependence on the value of the voltage developedacross resistor I5. If the transmission level at the input terminals ofamplifier 2 reaches, or exceeds, a predetermined am'- plitude level, theoutput of the harmonic genera' tor Q will be decreased.

In Fig. 2 there is shown a modification of theV invention whereinthemodulation signal input energy is applied to the primary winding of anaudio transformer IB. The transformer I9 is provided with a pair ofsecondary windings I9 and 20.

A tube 23 has its signal grid connected to the high side of winding 2t.The low side of winding 20.

is connected by an adjustable tap to a desired point on the biaspotentiometer resistor 25. The potentiometer 25 has one end thereofgrounded, while the opposite end may be connected to a direct currentsourcewhich will provide the negative bias for the grids of tubes 22 and23. In shunt with winding section 20 there is arranged a choke coil ZIwhose inductance value may be adjusted, as shown by th'e arrowtherethough.

The secondary winding I9 has one end thereof connected to the signalgrid of tube 22.\ The opposite end of the winding is connected to` apoint on resistor 25 which is at a less negative potential relative tothe point on the resistor to which winding 20 is connected. Hence, thegrid of tube 23 is given a more negative bias than the signal grid oftube 22. The cathodes of tubes 22 and 23 ae connectedin common through aresistor 30. The midpoint thereof is connected to ground tov provideadditional bias for the tubes, and to provide a commonly used humbalance. rIfhe screen grids and plates of tubes 22 and 23 are connectedto the positive terminal of a common energizing source. The plates ofthe two tubes are arranged in push-pull relation. The audio transformer2l couples the output circuit to the push-pull connected plates of thetubes 22 and 23.

The winding 20 is tuned by the retardation coil 2| so as to function asa high-pass filter. This is similar to the function of transformer Iwith the Ashunt retardation coil 3. Tube' 22 is quencies over 1000lcycles.

. '2,879,714 biased' normally, and it amplifis the signal energy in adistortionless manner. Thetube 23,

. which is more-negatively biased, functions to characteristics of ahigh fidelity audio amplifier. i In other words. this is thecharacteristic of the audio transmission system i 23 in Fig. 1, andi8-'22-24 in Fig. 2. Curve B represents the pass band characteristic ofa'good I. F. amplifier with a pass band width-not suiliciently broad toconvey 'all of speech intelligence. In other words, letitbe assumed thatthe `audio networks used in either of Figs. 1 or 2 are employed in aradio receiver of the superheterodyne type, wherein the I. F. amplifiershave pass band widths represented by curve B in Fig. 3. Such an I. F.am-

`piiiler is shown to have a pass band width of 4 kc., since some 2000cycles is passed on either side of the carrier. Obviously, in such caselthe reproducer would produce slightly unintelligible speech orjmusic.

The curve Crepresents the approximate characteristic of transformer l inFig. 1 as the result of the action of the retardation coil t. It will beseen from this characteristic that there is What I claim is:

l. A method of transmitting a narrow yband of modulation ysignal lfrequencies through a transmission network having a relatively widerfrequency pass band which includes, deriving from the narrow band ofsignal energy. those `frequencies located in the upper portion of thenarow band, producing harmonic frequencies from said upper portionfrequencies, and combining the harmonic frequencies with the aforesaidnarrow band signal energy thereby bto improve the intelligibilityofvsaid narrow band signal energy.

2. In combinationwith a high fidelity audio'I amplier network, ahigh-pass filter having input terminals connected to the input terminalsofI said amplifier. means for producing harmonic frequencies in theoutput of said high-pass filter,

and means for injecting said harmonic frepassed to the harmonicgenerator 9 of Fig. l fre- I The frequencies between 1200 cycles atpoint a and 2000 cycles at point b represent 800 cycles that is fed intothe harmonic generating tube 9 to be later reinserted to improve theintelligence impaired due to the narrow band I. F. amplifiercharacteristic. represents approximately 800 to 1000 cycles amplified,distorted and theharmonics re-insertedi to extend the apparent band passto about 5000.

In the broadcast band about 2200 cycles would be passed to the harmonicgenerator to extend therange from 15,000 to 17,000 cycles.

vFor partially restoring the higher harmonic content of standardbroadcast band intelligence,

the highfpass filter feeding the harmonic generator tube 9 would have acharacteristic similar to curve D of Fig. 3. same characteristics wouldbe used to build up the higher harmonic content in reproduction that isnormally lost in phonograph recording, and in sound-on-iilm recording.demonstrated that the present invention is' oi' particular value inrebroadcasting of .foreign Approximately the,`

The range between curves C and D It has been station transmissions, whenreduction of the rev ceived bandwidth is desired to a void interiorence,and when the'transmitted signal does not carry sufficient intelligence.'I'his is the case when a foreign station being received has a poorremote pick-up due to a poor line.

While I have indicated and described several systems for carryingmyinvention into effect, it

@will be apparent toone skilledv in the art that .my invention is by nomeans limited to the particular organization -shown and described, butthat many modifications may be made without departing from the'scope ofmy invention. as set forth in the appended claims. l

quencies into the output of said amplifier.

3. A method of improving the intelligibility of a narrow band of audiofrequencies which-includes deriving from the narrow band energy thosefrequencies located in .the upper portion of thenarrow band, convertingthe upper portion frequencies into harmonic frequencies, combining theharmonic frequencies with the original narrow band frequencies, and`controlling the' magnitude of the harmonic freuencies in response to theamplitude level of the narrow :band energy.

4. A method of improving the inteiligibility of a narrow band of audiofrequencies which includes deriving from the narrow band energy thosefrequencies located inthe upper portion of the narrow band, convertingthe upper portion frequencies into harmonic frequencies, separating theproduced harmonic frequencies from the selected fundamental frequenciesproducing same, combining the harmonic frequencies with the originalnarrow band of frequencies, and controlling the amplitude of theharmonic frequencies in response to the amplitude level of the narrowband energy. v

5. A method of transmittinga narrow band of audio signal frequenciesthrough a transmission network having a relatively wider audio-frequency pass band which includes-the steps of -deriving from thenarrow band of audio signal energy those audio frequencies located inthe upper portion of the narrow band, producing harmonic frequenciesfrom said upper portion frequencies, separating `the harmonicfrequencies, and combining the harmonic frequencies with the aforesaidnarrow band signal energy thereby to improve the intelligibilitylof saidnarrow band signal energy.

6. In combination with a high ndeluy audm amplier network, a high-passfilter having input terminals connected to the input terminals of saidamplifier', a. tube circuit for producing `harmonic frequencies in theoutput of said high.

pass filter, means for injecting said harmonic frequencies into theoutput of said amplifier, and additional means responsive to the audios18- nal level for controllingzthe harmonic amplitude.

7. A modulation signal transmission system comprising a main modulationsignal amplifier channel, an harmonic channel having input and outputterminals, means for applying to said ,harmonic channel input terminalsenergy from said main channel, a high-passniter in said harmonic channelto produce energy of frequencies in theupper portion of the mein channelfre' quency band, a distortion generator adapted to be fed with theoutput of said filter, said genery atorproducing harmonics means forconnecting said output terminals of the harmonic channel A amplitude.

8. A method of improving the intelligibility of a narrow band of audiofrequencies, which ine. A method or transmitting a. predetermined bandof frequencies througha transmission net eludes deriving from the narrowband energy such frequencies as are located in the upper portion of thenarrow band, converting the upper portion frequencies into harmonicfrequencies, combining the harmonic frequencies with the original narrowband frequencies, and controlling the amplitude of the harmonicfrequencies, in response to the amplitude level of the narrow bandenergy. lby rectifying Said narrow band irequency energy and using therectified energy to control the harmonic conversion.

work lhaving a relatively wider frequency pall Y band which includes,deriving from the bend those frequencies which are located in` the upperportion thereof, producing harmonic frequencies from said upper portionfrequencies, and combining the harmonic frequencies with the totalpredetermined band.

i0. A method of improving the intelligibility of a band of audiofrequencies which includes deriving from the band energy thosefrequencies located in the upper portion of the narrow band,

converting the upper portion frequencies into harmonie frequencies,separating the produced harmonic frequencies from the selected funda-`mental frequencies producing the same, and combining the harmonicfrequencies with tiie original narrow hand of frequencies. v

R. LEE HOHJNGBWORTH.

